Recent electronic apparatuses, such as mobile phones, tablet personal computers (PCs) and so on, are equipped with a liquid crystal display driver and various kinds of processors requiring a voltage higher than a battery voltage. A DC/DC converter is used to supply a proper power supply voltage to such devices.
One type of DC/DC converter control system is a hysteresis control system which provides higher load responsiveness than voltage mode and current mode control systems using an error amplifier.
FIGS. 1A and 1B are circuit diagrams showing a step-down DC/DC converter employing a hysteresis control system.
A step-down DC/DC converter 900a of FIG. 1A includes an output circuit 901, a driver 904 and a pulse modulator 906. The step-down DC/DC converter 900a stabilizes an input voltage VIN of an input line 902 to a predetermined voltage level, and supplies it to a load (not shown) connected to an output line 903. The output circuit 901 includes a switching transistor M1, a synchronous rectification transistor M2, an inductor L1 and an output capacitor C1. The output capacitor C1 includes an equivalent series resistor (ESR).
The pulse modulator 906 generates a pulse signal S1 whose duty cycle is adjusted such that an output voltage VOUT approaches a predetermined target level. The pulse modulator 906 includes resistors R1 and R2, a capacitor C2 and a hysteresis comparator 910.
The resistors R1 and R2 divide the output voltage VOUT. The divided output voltage VOUT is also called feedback voltage VOUT′. The hysteresis comparator 910 compares the feedback voltage VOUT′ with a threshold voltage VTH and generates the pulse signal S1 indicating a result of the comparison. The threshold voltage VTH transitions between two voltage levels VH and VL based on the comparison result. The pulse signal S1 has a high level for VH>VOUT′ and a low level for VL<VOUT′.
The driver 904 turns on the switching transistor M1 and turns off the synchronous rectification transistor M2 when the pulse signal S1 has a high level, and turns off the switching transistor M1 and turns on the synchronous rectification transistor M2 when the pulse signal S1 has a low level.
FIG. 2 is an operation waveform diagram of the step-down DC/DC converter 900a of FIG. 1A. At time t1, the pulse signal S1 transitions to a high level. At this time, the threshold voltage VTH transitions to the upper level VH.
During the period where the pulse signal S1 has a high level, a coil current IL increases and a voltage drop of the ESR increases accordingly. As a result, the output voltage VOUT increases and the feedback voltage VOUT′ increases accordingly. At time t2, when the feedback voltage VOUT′ reaches the upper level VH, the threshold voltage VTH transitions to the lower level VL and, at the same time, the pulse signal S1 from the hysteresis comparator 910 transitions to a low level.
During the period where the pulse signal S1 has a low level, the coil current IL decreases with time and the voltage drop of ESR increases accordingly. As a result, the output voltage VOUT decreases and the feedback voltage VOUT′ decreases accordingly. At time t2, when the feedback voltage VOUT′ reaches the lower level VL, the threshold voltage VTH transitions to the upper level VH again and, at the same time, the pulse signal S1 from the hysteresis comparator 910 transitions to a high level.
The above-described operation is repeated by the step-down DC/DC converter 900a. As a result, the feedback voltage VOUT′ is stabilized between VH and VL and the output voltage VOUT is stabilized between VH×(R1+R2)/R2 and VL×(R1+R2)/R2.
In the step-down DC/DC converter 900a of FIG. 1A, the voltage drop of the ESR of the output capacitor C1 is used as a ripple of the feedback voltage VOUT′. However, this may cause a problem that a switching frequency is affected by variations in the ESR and a power loss due to the ESR is not negligible.
A ripple injection type step-down DC/DC converter has been proposed to overcome the problem of the step-down DC/DC converter 900a of FIG. 1A. FIG. 1B shows a ripple injection type step-down DC/DC converter 900b employing the hysteresis control system.
The step-down DC/DC converter 900b of FIG. 1B further includes a ripple injection circuit 912 in comparison to the step-down DC/DC converter 900a of FIG. 1A.
The ripple injection circuit 912 receives the output signal S1 of the hysteresis comparator 910 or a pulse signal correlated with the output signal S1 and superimposes a ripple on an input side of the hysteresis comparator 910. More specifically, the ripple injection circuit 912 superimposes a voltage inclined in a positive direction with respect to the feedback voltage VOUT′ during the period where the output S1 of the hysteresis comparator 910 has a high level, that is, during the period where the switching transistor M1 is turned on and the synchronous rectification transistor M2 is turned off, and superimposes a voltage inclined in a negative direction with respect to the feedback voltage VOUT′ during the period where the output S1 of the hysteresis comparator 910 has a low level, that is, during the period where the switching transistor M1 is turned off and the synchronous rectification transistor M2 is turned on. As a result, a ripple is superimposed on the feedback voltage VOUT′ without using a ripple of the ESR.
An operation of the step-down DC/DC converter 900b of FIG. 1B will be described with reference to FIG. 2. At time t1, the pulse signal S1 transitions to a high level. At this time, the threshold voltage VTH transitions to the upper level VH.
During the period where the pulse signal S1 has the high level, the feedback voltage VOUT′ on which the positive-inclined voltage is superimposed by the ripple injection circuit 912 increases with time. At time t2, when the feedback voltage VOUT′ reaches the upper level VH, the threshold voltage VTH transitions to the lower level VL and, at the same time, the output S1 of the hysteresis comparator 910 transitions to a low level.
During the period where the pulse signal S1 has the low level, the feedback voltage VOUT′ on which the negative-inclined voltage is superimposed by the ripple injection circuit 912 decreases with time. At time t3, when the feedback voltage VOUT′ reaches the lower level VL, the threshold voltage VTH transitions to the upper level VH again and, at the same time, the output S1 of the hysteresis comparator 910 transitions to a high level.
The above-described operation is repeated by the step-down DC/DC converter 900b. As a result, the feedback voltage VOUT′ is stabilized between VH and VL and the output voltage VOUT is stabilized between VH×(R1+R2)/R2 and VL×(R1+R2)/R2.